Function to find boundary of an irregular sample?

Given a set of coordinates that form an irregular sampling area, is
there an R function to determine boundary points (coordinates defining
the limits of the area), either with or without user interaction ?

# for example, given the following irregular sampling area, how could
I define the boundary polygon?
library(SemiPar)
data(scallop)
plot(scallop$longitude, scallop$latitude)

Thanks.  --Dale